Enhance App.vue and ImageViewer.vue for improved styling and animation handling

- Added a custom variant for dark mode in App.vue to support Tailwind v4 features.
- Simplified the onEnter function in ImageViewer.vue by removing the done parameter, allowing Vue to manage CSS animation completion automatically.

src/App.vue

@@ -555,6 +555,9 @@ function saveSettings(customApi: string, newCustomCSS: string) {
 <style>
 @import "tailwindcss";
 
+/* Tailwind v4: 配置 dark 变体使用 .dark 类 */
+@custom-variant dark (&:where(.dark, .dark *));
+
 /* Ken Burns 动画效果 - 使用 CSS 动画实现更流畅的背景切换 */
 .ken-burns {
   /* 初始状态 - 稍微放大以便动画有空间 */

src/components/ImageViewer.vue

@@ -68,9 +68,9 @@ const imageStyle = computed(() => {
 })
 
 // 进入动画时播放音效
-function onEnter(_el: Element, done: () => void) {
+// 不声明 done 参数，让 Vue 自动等待 CSS 动画完成
+function onEnter() {
   playTransitionUp()
-  done()
 }
 
 // 监听图片切换